With respect to easily opened hermetically sealed packages, materials with which widely changing sealing strength can be attained by controlling the formulation of resin composition forming the package. Such materials are demanded because sealing strength will change, depending on the configuration of the package, the enclosed item and the heat sealed area.
When polypropylene is used as an adherend, the same kind of polypropylene is generally used as the heat sealing material. However, the exhibited sealing strength is so high that, although the sealability for enclosed item is excellent, opening the sealed area is difficult, causing the sealing material to be unsuitable for use in hermetically sealed packages which must ensure easy opening.
Japanese Patent Publication No. 7(1995)-17041 describes an easily openable unoriented hermetically sealed enclosure comprising a sealant of a composition composed of polypropylene and an ethylene/α-olefin random copolymer and an adherend of polypropylene. This publication is directed to an enclosure wherein a sealant having such a noncompatible phase structure that polypropylene is contained as a matrix phase while the ethylene/α-olefin random copolymer is contained as a dispersion phase is employed so that cohesive failure is caused at the time of peeling to easily open. In the noncompatible phase sealant capable of attaining the easy openness, although the sealing strength can be controlled by the configuration of dispersion phase, the dispersion phase is oriented at the time of film formation with the result that the sealing strength in the direction of flow (MD) at film formation is different from that in the direction perpendicular thereto (TD). Further, because polypropylene and the ethylene/α-olefin random copolymer have low melt tension, extrusion laminating of a mixture thereof brings about neck-in increase. Still further, inflation molding thereof possesses such a problem that disadvantages such as poor bubble stability would occur.
Therefore, there is a demand for the development of a sealant for polypropylene, which enables reducing the neck-in at extrusion laminating, which is excellent in the bubble stability at inflation molding, and which enables producing an easily opened hermetically sealed package exhibiting such a sealing strength as permits easily opening of a hermetically sealed package comprising polypropylene as an adherend, and further the development of such an easily openable hermetically sealed package.